Thomas Edison observes the Edison effect.13.2.1880

Thermionic emission is the heat-induced flow of charge carriers
from a surface or over a potential-energy barrier. This occurs because
the thermal energy given to the carrier overcomes the binding potential,
also known as work function of the metal. The charge carriers can be electrons or ions,
and in older literature are sometimes referred to as "thermions". After
emission, a charge will initially be left behind in the emitting region
that is equal in magnitude and opposite in sign to the total charge
emitted. But if the emitter is connected to a battery, then this charge
left behind will be neutralized by charge supplied by the battery, as
the emitted charge carriers move away from the emitter, and finally the
emitter will be in the same state as it was before emission. The
thermionic emission of electrons is also known as thermal electron emission.

The classical example of thermionic emission is the emission of electrons from a hot cathode, into a vacuum (also known as the Edison effect) in a vacuum tube.
The hot cathode can be a metal filament, a coated metal filament, or a
separate structure of metal or carbides or borides of transition metals.
Vacuum emission from metals tends to become significant only for
temperatures over 1000 K. The science dealing with this phenomenon has
been known as thermionics, but this name seems to be gradually falling into disuse.

The term "thermionic emission" is now also used to refer to any
thermally-excited charge emission process, even when the charge is
emitted from one solid-state
region into another. This process is crucially important in the
operation of a variety of electronic devices and can be used for electricity generation (e.g., thermionic converter, electrodynamic tether) or cooling. The magnitude of the charge flow increases dramatically with increasing temperature.